The present invention relates to a die for an underwater pelletizer, and particularly to a die for an underwater pelletizer in which the nozzle is constituted by a pipe, a heating jacket is provided at the outer periphery of the pipe, and a nozzle plate is provided at the tip of the nozzle.
Generally, pelletizer dies are widely used in the field of primary materials production in the synthetic macromolecular material manufacturing industry. Recently, however, the demand for the synthetic macromolecular materials shows a tendency of differentiation into two directions, one directed to high viscosity materials and the other directed to low viscosity ones. Accordingly, it has become very difficult to produce, by extrusion, pellets having a good shape, which is an important function of the die. That is, with lowered viscosity, when a synthetic macromolecular material extruded from a nozzle opening of a die is pelletized by cutting in water, each pellet leaves a tail because of its low viscosity, resulting in a deterioration in the pellet shape to thereby lower its commercial value. To cope with this situation, generally, the cutter is strongly pressed against the die surface to cut the synthetic macromolecular material extruded from the nozzle, resulting in a problem in that the cutter and the nozzle portion at the die surface are extremely worn.
To solve this problem, conventionally, a hardened layer of a material such as tungsten-carbide alloy is formed directly on the nozzle portion at the die surface by a metal spray method, or a so-called hard tip made of a sintered hard alloy is provided. In either case, the device life-time is one year at the longest due to wear and it is necessary to perform repair by reforming the hardened layer by the metal spray method in the former case or by replacing the worn sintered hard material tip by a new one in the latter case.
An ordinary pipe die is arranged, however, such that a pipe having a nozzle hole at its center is inserted into a die body, with the opposite ends of the pipe being air-tightly welded to the die body and the intermediate portion of the pipe being arranged to function as the inner tube of a heating jacket, so that in operation a vapor of 180.degree.-280.degree. C. is passed through the jacket portion to prevent the temperature of the synthetic macromolecular material being extruded from falling, while the hardened layer portion at the die surface is cooled by warm water at 60.degree.-80.degree. C. for solidifying and transporting the cut-off pellets. Further, bending stress is repeatedly imparted to the die due to the extruding force of 50-150 Kg/cm.sup.2 in extruding the melted synthetic macromolecular material.
By being arranged in the manner described above and acted on by bending stress, the conventional pipe die is disadvantageous in that if the metal spray or respray is repeatedly performed with a spray temperature which may rise to 1000.degree.-1100.degree. C., a crack may be generated at the welded portion between the die and each of the opposite ends of the pipe to cause leakage of heating vapor. Further, a bend may be generated in the nozzle land due to its expansion and contraction due to the temperature rise and fall in the metal spraying operation. The nozzle opening may be blocked in the respray operation and when an opening is again bored, the bored portion may be offset from the intial one so that a stepped portion may be formed on the nozzle land.
Although the jacket has the object of preventing the temperature of the synthetic macromolecular material passing through the nozzle of the die from falling below the melting point of the material, that is, 120.degree.-180.degree. C., the die surface temperature is apt to be lowered below 100.degree. C. because the die surface is exposed to the warm water at 60.degree.-70.degree. C., and at this time the temperature of the synthetic material extruded through the nozzle land is also lowered below its melting point so that the viscosity of the melted and extruded material in the nozzle land is raised. The synthetic macromolecular material is solidified in the worst case to make it impossible to cut or extrude.
To solve this problem, therefore, it has been required to suitably select the various metal materials to be successively used from the jacket portion to the die surface such that they have heat conductivities which decrease in this order. That is, the conventional die has a serious defect in that since the boundary film heat-transfer coefficient at the die surface is larger than that at the jacket portion due to the agitation caused by the cutter, if the respective heat conductivities of the various metal materials constituting the die are erroneously selected as to order, there occurs a phenomenon in that the temperature of the die surface is extremely lowered so that the moltern synthetic material is solidified in the nozzle land to make it impossible to produce well-shaped pellets of synthetic macromolecular material.